ABSTRACT

As the market for coated fabrics expands to applications with more complex geometries and loading conditions, a competitive edge can be gained by optimizing the selection and design of fabric substrate and coating materials.  Currently, the lack of understanding of the interaction between the fiber structure and the coating limits our ability to predict undesirable behaviors such as wrinkling, distortion, and tear. This combined experimental/modeling effort attempts to advance our fundamental knowledge of deformation mechanisms and parametric effects for these materials.    In this paper, the results of preliminary shear tests at elevated temperatures demonstrate the effect of coating viscosity on the fabric shear response. Macromechanical modeling of the fabric grab test geometry helps to explain the higher failure loads and failure behavior seen in these tests versus strip tests.   A brief discussion of anticipated future work in the area of experimental characterization, measurement technique development, and modeling is also presented.

INTRODUCTION:

Coated fabrics have wide applications in fields such as medical substrates, protective clothing, flexible membranes for civil structures, airbags, geotextiles, and industrial fabrics 1,2.   With cost a major factor, the competition from Asia and South America is fierce 3.  The key to US competitiveness is technical advantage -- high quality at a competitive price.  Manufacturers must understand how a material will behave in converting and end-use operations to optimize material and process selection.  While the emphasis in past studies on coated fabrics has been primarily on gas or liquid permeability, the mechanical properties are a fundamental component of the material design.  Flexibility, tensile strength, tear strength, and stretching and shearing response are all important factors.  Currently, selection of the optimal combination of textile materials, textile structures, and coating compounds for the anticipated loading condition is often based on the strength of the textile substrate alone.  Yet, it has been shown that the in-situ(coated) properties can differ significantly from that predicted based on the properties of the constituents prior to coating.  The discrepancies and the consequences are likely to be further exacerbated by the desire to utilize new textile substrates (woven and nonwoven) to obtain technically superior properties with less material.  The lack of understanding of the interaction between the fiber structure and the coating limits our ability to optimize material and textile structural configurations for coated fabrics under complex loading conditions.

The specific focus of this project is to address the following:

• Effect of textile structure and coating viscosity on coated fabric shear and stretching behavior.



•  Effect of coating migration on processing and end-use properties (e.g., effect of single and double-sided coating, effect of yarn impregnation).



• Effect of substrate and coating interaction on strength and stiffness.



• Development of advanced test and measurement techniques for characterizing coated fabrics under complex loading conditions.

BACKGROUND:

Models of coated fabrics typically represent the material as a non-linear elastic or inelastic material or as a visco-elastic material 2,4-6.   Some models predict the response of a fabric based simply on a rule-of-mixtures type approach.  More extensive models have been developed which use Classical Lamination Theory 2 or represent the combination of crimp interchange and elastic behavior based on results from uniaxial tests of the coated fabrics in the principal directions 7.  Some complex geometries have been investigated with finite element models 8. Experimentally, researchers have shown that the fiber substrate governs the strength, elongation, and dimensional stability, while the coating influences permeability, bonding, and abrasion properties 9,10. Of particular interest is that the shear behavior, tear strength, long-term performance, and adhesion depend on the interaction between the fiber substrate and the coating. 

These existing models of mechanical behavior have primarily focused on predicting response to axial and biaxial loadings of coated woven fabrics.  For axial and biaxial loading, the behavior is dominated by the fiber and yarn tensile response.  In contrast, loading off-axis to the fiber direction results in shearing, which involves a more complex interaction of fiber rotation, yarn compression at intersecting points, and flow or straining of the coating.  Because of the increased complexity, there has been little research conducted on the response under arbitrary loading paths and histories 2.   Even with membranes used for inflated structures, where the loading is primarily biaxial, the existence of seams results in a very different local stress distribution.  These stresses, combined with the potential weakness of the bond, lead to the common occurrence of seam failures.   This type of problem is just one example of the need to have a fundamental understanding of how the material deforms under more complex loading conditions.

EXPERIMENTAL APPROACH:

This project will have an extensive experimental portion in the initial stages, coupled with the development of a micromechanical model for coated fabrics.  The experimental component will be used to identify the governing mechanisms at different loading conditions.  This will include tensile, shear, and peel tests at a range of shear rates and temperatures. Other factors to be investigated include coating thickness, one-sided and two-sided coating, material types, textile structure, and gage length effects.

Sample 1 shows the experimental setup for measuring fabric shear under a range of shear rates and temperatures.   An infrared oven was constructed to enable rapid heating of the coated fabrics.   The trellis frame shear fixture is designed to apply a pure shear condition to the fabrics, with the warp and weft yarns aligned parallel to the sides of the fixture.  Sample 2 shows preliminary shear results for a plain weave fabric at temperatures ranging from room temperature to just below the melt temperature. Note that as the temperature increases, the shear force (i.e., the resistance to yarn rotation) decreases due to the drop in resin viscosity.

NOVEL MEASUREMENT TECHNIQUES:

One of the challenges of this research will be to measure the local deformation in the fabric during loading.  Combined with the global response, this can be used to determine some of the interaction effects and validate the model.  Because of the flexible nature of the fabric, this involves using some type of non-contacting method such as a video camera and image analysis software, but high resolutions are needed, as well as an efficient method of marking the fabric.  Such techniques have been used in the past by researchers, including one of the team members, but further refinement is needed to obtain useful correlation with the model.   Marking methods may include simple screen printing of the coated fabrics to quantify the global response. Various methods to measure the local yarn and fiber deformation and movement will be utilized, including (i) video/image analysis of a clear-coated fabric and (ii) OCT, optical coherence tomography.   A recent collaboration has been established with researchers at NIST who have been using this technique (OCT) to image fibrous structures.   Originally developed as a low-cost alternative to CAT scans, this technology can potentially be used by the textile industry forin-situ characterization of fibers and interfaces in textile structures.

Based on the experimental results, a combined micromechanical/macromechanical model of the coated fabric will be developed to represent the physics of the exhibited deformation.  The micromechanical model will start with the initial unit-cell geometry of the textile substrate.   Unit-cell deformation and flow/straining of the coating will be tied to an easily measurable parameter such as shear strain.   Additional experimental investigations will be conducted on the fiber-coating interface to incorporate this load transfer effect into the model.

 

FABRIC MODELING OF TEST CONFIGURATIONS:

To better understand and interpret the results of the various test configurations, some initial macromechanical modeling of the test geometries has been conducted.  In terms of specimen shapes, there are mainly two different types of tensile strength tests for woven fabrics, i.e., the grab and ravelled strip specimens. Although both specimens are designated as the ASTM standard (ASTM D 1682), the breaking load obtained for the grab specimen is greater than that for the strip method of the same fabric width inserted in the clamps.

There are some advantages to using the grab specimen. First, it is much simpler to prepare the samples, in comparison with the strip, especially the ravelled strip method. Also, the test result of a grab method corresponds more closely to load applications in practical use. It is extremely rare that stretching of a fabric is performed on a sample of the same width as the part where tensile forces are applied.  However, the strip or ravelled strip tests usually provide results that are easier to interpret.  The grab specimen is extensively used by textile industry, and the strip method is preferred by the research community. It is highly desirable therefore to establish the relationship between the testing results using the two different specimens of the same fabric.

There has been at least two attempts to accomplish this task, i.e., by Walen 11 in 1916  and by Eeg-Olofsson and Bernskiold 12. However, both methods have been largely empirical, and the physical implications are not clear in their treatments. So it is the purpose of this study to develop a much more rigorous theoretical analysis to establish the relationship between the testing results of the two type samples. This analysis and similar analyses for the other tests to be conducted on the uncoated fabric substrates and the coated fabrics will lead to proper interpretation of the experimental results.

A theoretical analysis was conducted to study the relationship between the strengths tested using grab and strip specimens respectively. The shear and tensile stresses distributions at the portion of the grab specimen not directly stretched are derived in 13,14. The total force carried by this portion of the specimen is calculated, which provides the difference between the test results of the two specimen types, and a parametric analysis is carried out to show the effects of the parameters related.

SUMMARY AND FUTURE WORK:

This research is focused on significant contributions in three areas:

(1) experimental characterization of coated fabrics; (2) development of experimental techniques for measuring deformation of fabrics; and (3) modeling of coated fabric mechanical behavior. In the first few months of this project, efforts have primarily been concentrated on identifying appropriate substrate/coating combinations and on developing robust methods of testing and of test analysis. Preliminary experimental results show that the effect of temperature on deformation behavior can be obtained using an infrared oven and shear frame fixture. Initial modeling efforts have made possible an improved interpretation of the difference between grab test and strip test results. Future work can be included a detailed experimental study of the effect of various parameters such as fabric construction, temperature, shear rate, and coating thickness on coated fabric shear behavior. Macromechanical modeling of the fabric stresses and strains will continue for the desired test configurations. Micromechanical modeling of the fabric deformation will be initiated, as will the development of experimental measurement tools and techniques. The outcomes of this project will lead to an improved fundamental understanding of the mechanical response of coated fabrics, as well as to the development of improved design guidelines for selection of substrate and coating compound.

 

REFERENCES:

1. Gillette, S. Mark, “End-use applications for coated fabrics,” J. Coated Fabrics, 22, 75-79 (1992).

2. Chou, T.-W., “Flexible composites,” J. Mater Sci, 24(3), 761-783 (1989).

3. Wuagneux, E. L., “How to coat & laminate a better mousetrap: converters and suppliers cite technology and cost as key drivers in this growing market,” Nonwovens Industry, Vol.29, No.4, p72, April 1998.

4. Thomas, S., and Stubbs, N., “Inelastic biaxial constitutive model for fabric-reinforced composites,”, J. Coated Fabrics, 13(3), 144-160 (1984).

5. Minami, H., and Nakahara, Y., “Application of finite-element method to the deformation analysis of coated plain-weave fabrics,” J. Coated Fabrics, 10(4), 310-327 (1981).

6. Kennedy, T.C., “Application of composite fracture models to coated fabrics,” J. Coated Fabrics, 24, 129 (1994).

7. Testa, R.B., and Yu, L.M., “Stress-strain relation for coated fabrics,” J. Eng. Mech., 113(11), 1631-1646 (1987).

8. Argyris, J., St. Doltsinis, I., and da Silva, V.D., “Constitutive modelling and computation of non-linear viscoelastic solids. Part II: Application to orthotropic PVC-coated fabrics,” Computer Methods in Applied Mechanics and Engineering, 98(2), 159-226, (1992).

9. Wilkinson, M., “Review of industrial coated fabric substrates,” J. Coated Fabrics, 26, 87-106 (1996).

10. Chen, Y., Lloyd, D.W., and Harlock, S.C., “Mechanical characteristics of coated fabrics,” J. Textile Institute, 86(4), 690-700 (1995).

11. Walen, E.D., “Comparison of Strip and Grab Methods of Testing Textile Fabric for Tensile Strength”, ASTM Proc., 16,Part 1, 370 (1916).

12. Eeg-Olofsson, T. and Bernskiold, A., “Relation between Grab Strength and Strip Strength of Fabrics”, Textile Research J., 18, 135 (1948).

13. Bassett,R.J., Postle,R. and Pan,N., “Experimental Methods for Measurement of Fabric Mechanical Properties: A Review and Analysis”, Textile Research Journal, 69, 866 (1999).

14. Pan, N., “The Relationship of the Tensile Strengths between Grab and Strip Specimens of a Fabric”, submitted (2000)